1. Field of the Invention
This invention relates to microminiature tips, and more particularly, to microminiature tips formed using semiconductor integrated circuit fabrication techniques.
2. Prior Art
Microminiature tips have application in a number of scientific and engineering technologies Scanning tunneling microscopes STMs and atomic force microscopes AFMs use very sharp tips which either contact or are placed in close proximity to a surface to be scanned. There are a number of other uses for microminiature tips, such as in profilometers. One important use is as a test probe point or microcontact for use in testing very high speed electronic components, for example, for making direct contact to a transmission line or conductor in a gallium-arsenide microcircuit, which has extremely small spacings between contacts points and which tolerates limited impedance mismatch at frequencies up to 150 gigaHertz. A sharp test probe tip is necessary when probing metals like aluminum which have a naturally occurring thin oxide layer that must be broken through to achieve a low resistivity contact. Various types of microsensors and microactuators would use microminiature tips. In medical and neurobiological applications microminiature tips are useful as ultraminiature electrodes or neuron probes. In an array, microminiature tips would be useful in a tactile sensor.
An atomic force microscope (AFM) scans over the surface of a sample in two different modes of operation In one mode, the contacting mode, a sharp tip is mounted on the end of a cantilever and the tip rides on the surface of a sample with an extremely light tracking force, on the order of 10.sup.-5 to 10.sup.-10 N. In the contacting mode of operation, profiles of the surface topology are obtained with extremely high resolution. Images showing the position of individual atoms are routinely obtained In the other mode, the tip is held a short distance, on the order of 5 to 500 Angstroms, from the surface of a sample and is deflected by various forces between the sample and the tip, such forces include electrostatic, magnetic, and van der Waals forces.
Several methods of detecting the deflection of the cantilever are available with subangstrom sensitivity, including vacuum tunneling, optical interferometry, optical beam deflection, and capacitive techniques.
The technical requirements for the cantilever-and-tip assembly include a number of different factors. A low force constant for the cantilever is desirable so that reasonable values of deflection are obtained with relatively small deflection forces. Typical values are 0.01-1000N/m. A mechanical resonant frequency for the cantilever which is greater than 10 kHz ia desirable to increase image tracking speed and to reduce sensitivity to ambient vibrations. Low force constants and high resonant frequencies are obtained by minimizing the mass of the cantilever.
When optical beam deflection is used to detect deflection of the cantilever, deflection sensitivity is inversely proportional to the length of the cantilever. Therefore a cantilever length of less than 1 mm is desirable.
For certain types of deflection sensing, a high mechanical Q is desirable and is achieved by using amorphous or single crystal thin films for fabrication of the cantilever.
In many applications, it is desirable that the cantilever flex in only one direction and have high lateral stiffness. This can be obtained by using a geometry such as V-shape which has two arms obliquely extending and meeting at a point at which the tip is mounted.
It is often required that a conductive electrode or reflective spot be located on the side of the cantilever opposite the tip. This is obtained by fabricating the cantilever from metal or depositing a conductor on certain portions of the cantilever.
The optimum shape of a tip for particular applications varies. For example, in STM and AFM applications a point contact is desired, while in electrical applications, a broader line contact, or knife-edge, is desired for reduced contact impedance. In STM or AFM applications, a sharp tip, that is, a protruding tip with a tip radius less than 500 Angstroms and which may terminate in a single atom, is also desired to provide good lateral resolution. A sharp tip has traditionally been very difficult to obtain in a consistent, reproducible manner.
In the prior art, cantilever arms were obtained by individually fabricating them by hand from fine wires One way of obtaining a tip portion was to etch a wire to a point and to bend the point to perpendicularly extend from the wire. Another way to obtain a tip was to glue a tiny diamond fragment in place at the end of a cantilever. Prior art cantilevers fabricated by using photolithographic techniques did not include integrally-attached sharp protruding tips. A rather dull tip was effectively obtained by using a corner of the microfabricated cantilever itself as a tip. Alternatively, a diamond fragment was glued by hand to the end of a microfabricated cantilever. The cantilever assembly of an AFM is relatively fragile and is virtually impossible to clean when it is contaminated by material from the surface being scanned so that frequent replacement is required.
Anisotropic etching of (100) silicon wafers to form pyramidal-shaped or knife-edged pits is discussed in an article by K. E. Petersen entitled "Silicon as a Mechanical Material," Proceedings of the IEEE, Vol. 79, No. 5, pps. 423-430, May 1983. This article also refers to an article by D. A. Kiewit entitled "Microtool fabrication by etch pit replication," Rev. Sci. Instrum., vol.44, p. 1741, 1973, which discloses using thermally grown silicon dioxide as a mask for selective etching of pits in a (100) wafer and which discloses filling those pits with electrolessly deposited nickel-phosphorous.
Currently, technologists are attempting to microfabricate STMs and AFMs using the microfabrication techniques which are compatible with standard fabrication processes used in the silicon semiconductor integrated circuit industry. Their goal is to mass-produce very precise, very reliable sensors which have minimal thermal drift, signal loss, and low noise characteristics by taking advantage of the inherent low mass, high resonant frequencies, and low thermal drift characteristics of microfabricated devices. In addition, these microfabricated sensors can be integrally combined with electronic circuitry fabricated with the same processes.